The current renewal application investigates the interaction between altered muscle mitochondrial function and mitochondrial proteome homeostasis contributing to insulin resistance. We will compare insulin resistant (IR) pre-diabetic abdominally obese people with insulin sensitive (IS) lean people to determine whether inefficient mitochondrial coupling and associated ROS (reactive oxygen species) emission are higher in IR, causing irreversible damage to mitochondrial proteins. We will investigate whether oxidatively damaged mitochondrial proteins are efficiently degraded in IR and replaced with new copies. Based on our preliminary data, we hypothesize that reduced insulin action in IR people when combined with increased free fatty acids (FFA) enhances synthesis of fatty acid intramyocellular transporter proteins while inhibiting synthesis of mitochondrial ?-oxidation proteins. Increased capacity to transport FA into muscle and reduced capacity for ?-oxidation could result in reduced mitochondrial coupling and phosphorylation efficiency. Our preliminary data supports that aerobic exercise (AE) and metformin independently enhance mitochondrial efficiency and reduce ROS emission in IR people. However, AE increases mitochondrial respiration while metformin reduces it. Hence, it remains to be seen whether metformin adversely affects endurance capacity or more efficiently channels mitochondrial respiration towards increased ATP production and reduced ROS emission. It also remains to be understood how the mitochondrial functions and proteome homeostasis are altered by high insulin, glucose, FFA, and amino acids (AA) in IR states. We will first perform a baseline comparison of IR people (n=48) and IS lean people (n=12) in the postabsorptive state and while on similar high levels of physiological insulin, FFA, glucose, and AAs. The IR participants (n=12 each) will then be randomized to 3 months of one of the following: (A) exercise [high intensity interval training plus placebo] (B) metformin (C) exercise plus metformin and (D) placebo. Baseline studies (pre-) are repeated post-intervention. We propose that exercise enhances mitochondrial respiration and ATP production but reduces ROS emission in IR people. We also hypothesize that metformin reduces mitochondrial ROS emission and enhances mitochondrial efficiency without increasing respiration. Adding metformin to exercise is hypothesized to attenuate exercise-induced mitochondrial respiration although a more efficient mitochondrial coupling will result in reduced ROS emission and similar ATP production as exercise alone. We propose that exercise maintains protein quality by increasing muscle mitochondrial protein turnover whereas metformin maintains protein quality with less ROS mediated protein damage and no increase in energy consuming protein turnover. The questions addressed in the study, especially the potential benefits on energy economy by combining metformin and exercise, has obvious clinical implications. Finally, the mechanistic outcomes of the proposed studies may offer potential opportunities for targeted drug discoveries.